So before Mahouka becomes this season's SAO or Attack on Titan I'd just like to share something that frequently pops up on 2chan when discussing it
Store clerk: "3000 JPY pleas... What, you're giving me 10000 JPY!? Why?!"
Customer: "What, you don't know? I'll explain it to you. When paying at the cash register, when presented with a bill whose value is larger than the actual amount needed, the store side of the transaction needs to prepare the « difference » between that and the price of the merchandise I'm « buying ». In technical jargon, this is called « change ». Well, normal students usually only use 1000 JPY or 5000 JPY bills, so it's pretty normal for you to be surprised if I present you with a 10000 JPY bill all of a sudden.... Heh."